/**
 * 减法
 * @param a string|number 被减数
 * @param b string|number 减数
 * @return string
 */
$.bcSub = function (a, b) {
	let _a = (a+='').charAt(0)==='-',
		_b = (b+='').charAt(0)==='-';
	if(_a!==_b){
		if(_b) return $.bcAddDot(a,b.slice(1));
		let rtn = $.bcAddDot(a.slice(1),b);
		return rtn==='0' ? '0' : '-'+rtn;
	}
	if(!_a) return $.bcSubDot(a,b);
	return $.bcSubDot(b.slice(1),a.slice(1));
};
